Electric discharge device



May 17, 1938. D. G. PRINZ ELECTRI C DISCHARGE DEVICE Filed June 29, 1937 IIVYENTWR.

Patented May 17, 1938 PATENT OFFICE ELECTRIC DISCHARGE DEVICE Dietrich Giinther Prinz, Wembley, Middlesex,

England,

assignor to The General Electric Company. Limited, London, England, a company of Great Britain Application June 29, 1937, Serial No. 150,897

In Great Britain July 3, 1936 8 Claims. (Cl. 175354) This invention relates tomeans for operating electric discharge devices, such as mercury vapor devices, of the type having a cathode from which vapor is generated during operation, from an approximately sinusoidal A. C. supply, so that the arc, which has to be started in each cycle, can be started at a controlled point in the cycle.

It has been proposed to start an arc to the mercury cathode of a mercury arc device by application of high voltage to an inner auxiliary electrode. This involves, however, either inconveniently high voltages or frequencies or vapor pressure, or special arrangements, such as auxiliary hot cathodes.

It has been proposed also to start an are by means of an external ignition electrode, for instance, a metal foil, fixed on the' glass wall near the mercury surface. In this case, a cathode spot is very easily formed on the edge of the mercury probably due to the abnormally high electrical field arising at this sharp edge.

This method of ignition, however, has some considerable disadvantages.

Since the main anode circuit always contains a certain self inductance, theanode current will increase, not suddenly, but at a 'finite rate. On the other hand, a minimum current of about 1 or 2 amps. is required for maintaining the oathode spot. Therefore, since the duration of the spot is very short, the spot may be extinguished before a current sufiicient to maintain it develops.

Another disadvantage of forming a cathodespot solely by means of an external electrode is that, if the anode be placed far from the cathode (forinstance, to protect it against evaporated or sputtered mercury), the main discharge may not,

be able to start from the cathode spot to the main anode, owing, for example to negative charges formed on the glass walls or other'insulators between cathode and anode.

, The object of the present invention is to proby this voltage until the main discharge is established.

Certain embodiments of the invention will now be described by way of example, with ref-' erence to the accompanying drawing. Of these, Figures 1, 2 and 3 show, by way of example, various circuits for the production of the ignition peak voltage and for the supply of voltage to the auxiliary electrode. by way of example, some arrangements and shapes of the auxiliary electrode.

In Figure 1, the discharge device consists of a glass envelope I, containing a mercury pool 2 with a cathode lead 3, an anode 4 and an inner auxiliary electrode 5. This electrode 5 may consist of a simple wire. sealed throughthe glass wall, or of a ring (as shown), or it maybe of any other suitable shape, of which examples are Figures 4 and 5 show,

mercury edge, but preferably of a metal foil fixed on the glass and surrounding the whole mercury pool.

Both the auxiliary electrode 5 and the external electrode 6 are connected with one terminal of the high voltage winding of a transformer l,

the other terminal of the said high voltage winding being connected with the cathode lead 3. In the connection between the electrode 5 and the said winding, there may be inserted a resistance I, if desired. 7

The primary winding of the transformer 1 is connected with the A. C. source 8 through a resistance 9 and a gridglow tube In, the grid of which is connected with its cathode, through the secondary winding of a transformer II (which is preferably a peak transformer of the saturated iron core type) and a D. C. source It.

The primary winding of the transformer ll is fed by an A. C. source ll of the same fre quency as the A. C. source 8 and as the, supply to the main circuit. This main'circuit is shown in Figure 1 as an A. C. source a connected through a transformer b with the electrodes 3 and l of the discharge devicel in series with a D. C. load c. The main circuit need not be as shown in Figure 1, but may be of any kind usual with vapour electric devices of the type specified. The secondary voltage of the transformer II, in cooperation with the D. C. source l2, renders the grid glow tube It) conductive, in known manner, at a definite moment of each cycle, so that an impulsive current then flows through the primary of the transformer 1. This impulsive current produces a high voltage on the secondary of the transformer I, so that the external electrode 6 forms a cathode spot on the edge of the mercury pool. At the same moment, a discharge is started between this cathode spot and the inner auxiliary electrode 5; this discharge is maintained for a finite time by the magnetic energy stored in the transformer I. If the transformer be suitably designed, this energy will be great enough to -maintain the discharge to the electrode 5, until the main discharge to the anode 4 is established.

The A. C. sources 8 and I3, or the A. C. source I3 only, may be provided with phase shifting devices of known kind, so that the moment of ignition in each cycle may be varied in a manner well known in connection with all grid controlled converters and thus the mean current varied.

Figure 2 shows another example of a circuit according to the invention. Reference numerals I8 have the same meaning as I-B, in Figure 1. The peak voltages of the A. C. source 8' cause sparks to pass across the spark gap 9, thus charging the condenser I and causing a heavy current impulse to flow through the primary winding of the transformer I; a voltage peak therefore ocours in the secondary winding of this transformer and a cathode spot is formed by the external electrode 6 as described above.

It may be pointed out that two sparks occur in each cycle, thus producing one positive and one negative voltage peak on the external electrode 6'. Only the positive peak is used to form the spot. By inserting a second transformer in the circuit I, 8, 9, III, the second spark may be used for forming another positive voltage peak, so that another dischargedevice may be ignited alternately with the device I. This arrangement may be useful in known symmetrical rectifier circuits, fed by 2-phase, 6-phase or 12-phase supplies. If the A. C. source 8' be provided with a phase shifting device, only one such device is required for each pair of rectifiers.

In Figure 2, a D. C. source I4 is connected through a resistance IS with a condenser I6, this condenser in turn being connected with the auxiliary electrode through an impedance I1.

As soon as the cathode spot is formed by the external electrode 6, a discharge takes place between the mercury cathode 2 and the auxiliary electrode 5, the duration of said discharge depending on the values of the condenser I6, the

resistance I5, the impedance I1 and the voltage of the D. C. source I4. If these values be chosen suitably, the auxiliary discharge will be maintained until the main discharge has been established.

In the succeeding non-conductive part of the cycle, the condenser I6 is recharged by the D. C. source I4 through the resistance I5. This resistance must be high enough to prevent a continuous arc discharge taking place between the auxiliary electrode 5 and the cathode.

The impedance I'I need not be a pure ohmic resistance; it may be replaced by a self inductance; indeed it has been found that in this way a more uniform duration of the auxiliary discharge can be obtained.

In Figure 3, the reference numerals I"-8 and I4"-I'I have the same meaning as I'--8 and I4-I'I in Figure 2; but the transformer 'I" is a peak transformer of the saturated iron core type, producing an impulsive voltage. Its secondary winding may be connected directly between the external electrode 6 andthe cathode 3"; its peak then must be made sharp enough and high enough to produce a cathode spot on the mercury. But preferably, as shown 'in the figure, the voltage on the external electrode 6 is made to rise still more sharply by inserting a spark gap I8 between the secondary winding of the transformer I" and the electrode 6". The spark gap I8 may consist of a point separated from a plate by a distance such that only the As mentioned above, it may be sufficient sometimes to form the auxiliary electrode as a simple wire sealed through the glass wall; in other cases,

it is preferable to give it a form that enables it to perform other functions as well.

In Figure 4, for example, the auxiliary electrode is formed as a baffle or screen protecting the anode from the vapour stream and the sputtering from the cathode spot. In this figure, the glass envelope 2| contains an anode 22, sealed and supported at 23, and an auxiliary electrode 24, sealed into and supported by a foot-tube and pinch 25. The annular depression formed by sealing the foot-tube to the glass envelope is filled with mercury 28, for which a leading-in wire 21 is provided. The annular form of the mercury pool has the advantage of increasing the length of the mercury edge for a given amount of mercury; a metal foil forming the external electrode can be applied not only on the outer side 28, but

also on the inner side 29 of this mercury pool. Furthermore, its presence on this inner side makes it easy to cool the mercury (for example by a blast of air); cooling is known to promote the formation of cathode spots.

The diameter of the auxiliary electrode 24 is greater than that of the anode 22 and that of the mercury pool 26, so that the anode is protected from the vapour stream produced by the cathode spot as well as from any sputtered mercury.

In Figure 5, another shape of the auxiliary electrode is shown. In this figure, the glass vessel 30 contains in aside arm 3| an anode 32 supported by a rod 33 sealed in at 34, the said rod being protected by a quartz sheath 35 held in position by washers 36 and a spring 31. 38 is the mercury pool, and 39 is the cathode lead. The auxiliary electrode 40,'supported on a lead sealed in at 42, is formed as a grid surrounding the anode 32. This arrangement facilitates the starting of the discharge between the cathode and anode; and, if the discharge tube be used as a rectifier, it also decreases the risk of back firing during the inverse half cycle. When using a circuit according to Figure 1 or 2, the potential of the auxiliary electrode never differs very greatly from that of the cathode during the inverse period; the electric field tending to produce a reverse discharge is confined to the space between the anode and auxiliary electrode. On the other hand, a back discharge between the anode-and the auxiliary electrode is improbable because these electrodes are near together and, in the circult according to Figure 1, because of the resistance of the transformer windings and the additional resistance ll inserted in the lead to the auxiliary electrode 5.

I claim:-

1. In a mercury vapor discharge device, a tube having an anode and a cathode, the cathode comprising a pool of mercury from which vapor is generated during operation, a main circuit carrylng alternating current of known frequency connected for discharge between the anode and cathode of said tube, said main circuit including a direct current load, an external electrode arranged to cooperate with the mercury pool of the cathode in the formation thereon of a oath.- ode spot, a circuit between said external electrode and the cathode carrying an impulse current for establishing a positive potential on the external electrode during each cycle of the alternating current in the main circuit for forming the cathode spot on the mercury pool, an auxiliary electrode mounted within the tube between the anode and cathode for cooperation with the cathode spot, and circuit means arranged to provide an impulse current for discharge between said auxiliary electrode and the cathode spot for maintaining the cathode spot until the main circuit discharge is established.

2. The mercury vapor discharge device as in claim 1 wherein the current for providing the impulse discharge between the auxiliary electrode and the cathode spot is derived from the circult which is arranged to form said cathode spot on the mercury pool.

3. The mercury vapor discharge device as in claim 1 wherein the impulse current for forming the cathode spot on the mercury pool is provided by the secondary of a transformer the primary of which is supplied with unidirectional impulse current produced by passing an alternating current of proper frequency through a glow tube rectifier.

4. The mercury vapor discharge device as in claim 1 wherein the impulse current for forming the cathode spot on the mercury pool is procurrent derived from an alternatingcurrent of proper frequency and the impulse current for the discharge between the auxiliary electrode and the cathode spot is provided by a branch circuit from the secondary of said transformer.

5. The mercury vapor discharge device as in claim 1 wherein the impulse current for forming the cathode spot on the mercury pool is provided by the secondary of a saturated core transformer the primary of which is supplied with impulse current of proper frequency from an alternating current source with a condenser and spark discharge in circuit.

6. The mercury vapor discharge device as in claim 1 wherein the impulse current for forming the cathode spot on the mercury pool is provided by the secondary circuit of a saturated core transformer which circuit includes a undirec-' tional spark discharge, the primary of the transformer being supplied from an alternating current of proper frequency.

7. The mercury vapor discharge device as in claim 1 wherein the impulse current for the discharge between the auxiliary electrode and the cathode spot is provided by a condenser arranged to be charged from a direct current source and to be discharged between the auxiliary electrode and the cathode spot with the formation of the cathode spot.

8. The mercury vapor discharge device as in claim 1 wherein the impulse current for forming the cathode spot on the mercury pool is provided by a transformer the secondary of which is arranged to supply a unidirectional impulse current produced from an alternating current of proper frequency and the impulse current for the discharge between the auxiliary electrode and the cathode spot is provided by'a condenser arranged to be charged from a direct current source and to be discharged between the auxiliary electrode and the cathode spot with the formation of the cathode spot.

DIETRICH GUNTHER PRINZ. 

